The process control and the transport industry employs process parameter gauges to monitor process parameters associated with substances such as solids, liquids and gasses in industries directed to chemicals, petroleum, pharmaceuticals, food, etc. Process parameters include pressure, temperature, flow, level, chemical composition and other properties. For measuring a level of a material contained in a tank, radar level gauge systems are often used. These systems normally employ a transmitter to transmit microwave energy, an antenna to guide the microwave energy, and a receiver to receive, as a reflection from the level to be gauged, a fraction of the transmitted microwave energy. The antenna here provides for noninvasive measurements.
As tank content, especially in the transport industry, generally are exposed to motion, the material in the tank can potentially come in contact with the antenna, possibly creating a thin layer of deposit on the antenna. Further, damp and condensate may easily generate deposits on antenna surfaces. The dielectric antenna parts typically have a necessary seal function and are penetrated by the radar signal and thus any attenuation by the passage will degrade the performance. In most cases materials like polytetrafluoroethylene (PTFE), which are difficult to wet by condensation, are used as the condensation on such a surface will form droplets with minor influence on the radar signal as long as the droplets are small (circumference<<wavelength). With deposits from the material in the tank, a continuous layer or a thicker layer can be formed and due to the dielectric properties of water (very high dielectric constant and high losses) even a few tenths of a mm may more or less stop the function. It is easily understood that this layer of deposit will create quality problems in relation the measurements, such as initially a small attenuation of the reflected microwave energy, and eventually a complete attenuation of the reflected microwave energy. There is therefore a need for a method to in advance indicate whenever such and similar problems occur.
U.S. Pat. No. 5,614,911 addresses this problem by using a method for level measurement on a radar basis, where microwaves are emitted by means of an antenna to the surface of a material in a container and the echo waves reflected from the surface thereof are received. To recognize the formation of a deposit and/or other trouble, such as e.g. damage to or loss of the antenna, an undisturbed echo function corresponding to an undisturbed measurement is obtained and stored prior to performing the measurements. In each measurement the actual echo function is compared to the stored undisturbed echo function. When due to the comparison, deviations between the two functions are found in the antenna region and the proximity zone, these deviations are evaluated to recognize the formation of a material deposit and/or other trouble. However, in many situations this known method is unsatisfactory, and/or unreliable. For example, since this method compares an “ideal” reference with a “real”, subsequent value (measured continuously) environmental variations occurring there between, such as temperature variations, will not be correctly taken into account. Hence, false alarms indicating deposit and/or other trouble may occur.
Another document, U.S. Pat. No. 5,689,265, also proposes a level measuring device using microwaves, wherein the formation of deposits of the material on the antenna or further trouble conditions, such as damage to the antenna or the loss thereof, is detected. To this end, the level measuring device comprises an arrangement which compares a section of the echo function originating from a reference reflection point in the antenna or in the vicinity of the antenna with a predetermined threshold value and produces a signal indicating the existence of a state above or below the said threshold value. The reference reflection point may be constituted by a part of the antenna or a separate reference reflector mounted in the antenna or in the vicinity thereof. A problem with this system is that the antenna has to be constructed in such a way that this special reference reflection point is present in the antenna area, thereby leading to complicated and costly constructions. Furthermore, the use of close reference points has shown to be unfavorable as the close distance resolution is relatively unreliable.
There is therefore a need for an improved method for in advance indication of whenever a problem regarding quality of measurement relating to deposit on an antenna exists. Hence, the object of the present invention is therefore to provide an improved method, for determining quality of measurement in a radar level gauge system, that provides a solution to at least some of the above-mentioned problems.